In the practice of orthodontics, tension wires are frequently used to change the position of the teeth within the mouth. These wires are affixed to individual teeth by either enclosing the tooth in a band or by cementing a metal or ceramic bracket to the tooth using an adhesive, known in the art and referred to herein as an "orthodontic bracket adhesive". Brackets are currently more popular, as bands tend to promote dental caries in the region adjacent to the band, and are also somewhat inconvenient for the orthodontist, as they must be stocked in many different sizes.
A number of orthodontic bracket adhesives are known. For example, U.S. Pat. Nos. 4,435,160 and 4,695,251 to Randklev describe an orthodontic bracket adhesive composition which contains a non-toxic filler having an Mohs hardness of less than about 4.5, a polymerizable resin system and, optionally, an adjuvant such as an inhibitor, stabilizer, or pigment. The method of using the composition requires preliminary cleaning and etching of the tooth surface. U.S. Pat. No. 4,134,929 to Stoakley et al. describes an orthodontic bracket adhesive based on allyl 2-cyanoacrylate. Light-curable adhesive compositions are described in U.S. Pat. Nos. 4,457,818 and 4,689,015 to Denyer et al.; these compositions are stated to be useful, inter alia, as orthodontic bracket adhesives. The compositions contain a polymerizable composition of a polymerizable vinyl urethane prepolymer, ethylene (or triethylene) glycol dimethacrylate, and a catalyst system which includes compounds such as dimethylaminoethyl methacrylate and camphorquinone. U.S. Pat. No. 4,479,782 to Orlowski et al. also relates to a light-curable orthodontic bracket adhesive which contains an aromatic acrylate, an aliphatic acrylate, an .alpha.,.beta.-diketone, a tertiary amine, and, optionally, fillers, stabilizers, or the like.
The recent advances in light-curable adhesive compositions, exemplified by the Denyer et al. and Orlowski et al. patents, have given rise to a bonding procedure which is simpler and far more rapid than earlier systems involving chemical curing. Light-curable adhesives, for example, enable manipulation and tying of archwires immediately after application and curing. However, all of the bracket adhesive systems of which applicants are aware, including the relatively new light-curable systems, require application of primer prior to application of the adhesive. Application of primer requires an extra step in the bonding procedure, additional time, and a sensitive technique. A thin and uniform primer coat has to be carefully placed; otherwise, the brackets may drift and consistent high bond strength may not be achieved. Furthermore, after completion of curing, tooth surfaces adjacent the brackets may remain tacky due to unpolymerized monomer in the adhesive composition.
There is thus a need in the art for a light-curable orthodontic bracket adhesive which does not require pre-treatment of the tooth surface with primer, but which nevertheless provides good adhesion with a minimum of bracket drift.